Otto, I split off your post to a separate topic to make it easier to find for others with the same question.
Now on to your question, you insert the ‘rel="lightbox" inside your hyperlink as follows:
[code:1ljk8p74]
<a title="The title for display" rel="lightbox-groupname" href="http://link.to.image"><img src="http://link.to.thumbnail"/></a>
[/code:1ljk8p74]
Hope that helps, if not let me know. I’m planning on eventually getting a userguide of sorts written, but for now features are more important, I think.